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July 16, 1929. I V 'N BR NK 1,721,141

APPARATUS FOR CONVERTING SOUND WAVES INTO ELECTRIC WAVES Filed April 3, 1924 E. 4- Q's wanker.

01m VON BRONK OTTO VON BBONK, OF BERLIN, GER-MANY, ASS ZGNOR-TO GEQELLSOHAFT DRAHT- LOSE TELEGRAPHIE IVI. B. I'l, OF BERLIN, GERMANY, A G'QRPOEATION OF GER- MANY.

APPARATUS FOR CONVERTING SOUND WAVES INTO ELECTRIC HAVES.

Application filed. April 3, 1924, Serial No. 703,881, and in Germany May 25, 195-33.

LM'y invention relates to means for converting sound waves into electric waves.

It has for its object the provision of such an arrangement, which, by the use of what may. be called a thermal microphone, will provide accurate and large current variations from a given sound wave.

Other objects will be apparent from the following description, when considered with the accompanying drawings, in which:

Fig. 1 shows a circuit containing athermal microphone according to the invention.

Fig. 2 shows a modification in which the thermal microphone is coupled directly to a thermionic valve.

Fig. 3 shows a further modification in which a lVheatstone bridge is employed.

Fig. 4 shows an arrangement in which the differential effects of the bridge shown in Fig. 3 is amplified by means of a thermionic valve before being utilized.

The invention relates to an arrangement for converting sound waves into electric oscillations which differs from other arrangements in that the sound wave is not distorted by natural oscillations of the system. The invention is based upon the principle that the temperature and therefore also the conductance of a glowing body is altered by sound waves being impressed on it. These. variations of resistance alter according to the shape of the sound waves, the voltage of V a circuit which variations may be coupled to the grid of a thermionic valve.

Referring to Fig. 1, 14 is a thermal microphone comprising a frame 1, consisting, for instance, of soap-stone on which the body '2, which is to be rendered incandescent is fastened in zigzag'shape. This body 2 may consist, for instance, of a thin platinum wire or an alloy of high resistance coefficient. The ends 3 and l of the incandescent wire are connected in circuit with the regulating resistance 5, the current source 6, furnishing the heating current and a winding 7 of trans former 8. The other winding 9 of trans former 8 is connected to the grid 10 and cathode 11 of a three element thermionic tube 12 which also contains plate 13.

It can easily be seen that the sound waves falling upon incandescent wires 2 vary the resistance of these wires in accordance with their form in the well-understood manner. The variations of current in winding 7 will induce similar variations in winding 9 of transformer 8 which variations are impressed on the filament l1 and grid 10 of tube 12. The tube 12 will. amplify those variations and the amplified variations can be led off from the plate 13 and filament 11.

Referring to Fig. 2 the thermal microphone 2% is connected in circuit with regulating resistance 25 and battery 26 which supplies the heating current for microphone 24-. Microphones 2 tmay be similar to micro phone 1%, describedv in connection with Fig. 1. Across terminals 33 and Set of microphone is connected the grid 20 and filament 21 respectively of thermionic tube 22. Battery 35 is an auxiliary grid battery to obtain conditions suitable for the best working of tube 22. Across plate 23 and filament 21 of tube is a circuit including B-battery 3(5 and receiver 37.

It ill easily be seen that the voltage across the terminals 38 and 34 will vary according to the sound waves impressed on the incandenscent wires 32. These variations are impressed on the grid 20 of tube 22 and will produce similar but much larger variations in the receiver 87 in accordance with the amplifying action of the tube 22. The incandescent body 32 may consist of a metal or of a conductor of negative tempera ture coefficient as. for instance, carbon or the rare materials of the Nernst glowers.

In order to increase the effect, two kinds of materials, one with negative and the other with positive tempe ature coefficients can be used together for one grid, the single rods of which consist alternately of these materials. hen such a grid is connected in a VVheatstone bridge connection to a receiver, the differential effect of the alteration of resistance acts on the receiver.

Such a connection is schematically shown in Figure 3. Two arms of the bridge contain two bodies 43 and 44; of positive and negative temperature coefiicients respectively heated by the common battery 46 across the regulating resistance 45, which makes up the other two arms of the bridge. The diagonal branch of the bridge contains the telephone 47. It is obvious that if the resistances 43 and 44 are altered simultaneously in opposite sense, the balance of the bridge is disturbed and a differential effect occurs in the telephone.

In Fig. 4 the differential effect'obtained by utilizing elements 43 and 44 in the bridge having respectively positive and negative temperature coefiicients is amplified by t-l'iermionictube 52 before utilization. The amplified potential differences may be used for actuating a telephone 57 or if desired may be fed to the modulator of a radio broadcast transmitter.

My invention is not limited to the specific embodiments described above but is entitled to all modifications that fairly fall within the scope o't the following claims.

I claim:

1. In an arrangement for converting sound waves into electric oscillations, a plurality or heated elements whose electric conductivity is varied by the action of sound waves, said heated elements COIlSlStlllg. of two dissiinilar substances of separate positive and negative temperature coefiicients' and means to lncorp'orate said substances into a brldge circuit, whereby a differential effect is obtained. v

2. In a system for converting sound waves into electrical oscillations, a bridge" circuit having in two of its arms heated elements having positive and negative temperature coefiicients respectively," said elements being arranged so that sound waves impinging simultaneously on both oftliein cause unbalancing of the bridge and also being arranged so that the differences in potential across the bridge due to the unbalancings eflicients, the conductivity of said elements being variable under the action of soun'cl waves, said elements being connected together so as to obtain a differential effect when afl'ccted by sound Waves, and being so connected together as to allow the coupling of means responsive to current changes, thereto.

OTTO VON BRONK. 

